Hydraulic power steering system for motor vehicles

ABSTRACT

A vane cell pump is provided for a hydraulic steering system for motor vehicles for pressure supply, a flow valve whose opening cross section is at least substantially closed during cold start operation being arranged downstream of said vane cell pump on the delivery side.

The invention relates to a hydraulic power steering system for motor vehicles according to the preamble of claim 1.

Power steering systems of the abovementioned type are known from DE 42 20 624 A1 and are provided, in the junction from the high pressure pump to the steering valve, with a flow valve whose opening cross section, in order to change the booster effect across the vehicle speed, can be varied via an appropriately activated, electromagnetic actuator.

Furthermore, DE 195 29 807 A1 has disclosed a vane cell pump, downstream of which on the delivery side there is arranged a 3-way flow regulating valve which opens the delivery to the consumer only when a certain minimum pressure is present. This is intended to improve the cold start behavior of the vane cell pump. It is assumed here that sufficient delimitation of the working spaces with respect to one another is not achieved and therefore short circuit effects occur under cold start conditions as a result of insufficient radial pressure loading. The delivery capacity is reduced as a result and the disadvantage of the reduced delivery capacity and of the slower build-up of pressure as a result are to be compensated for by the outflow path on the delivery side being closed by the flow regulating valve. The flow regulating valve configured with regard to this is complicated in construction terms.

The invention is based on the object of configuring a hydraulic steering system of the type mentioned in the introduction in such a way that said hydraulic steering system satisfies the increased comfort requirements, to be precise with low energy input and without increased structural complexity.

This is achieved according to the invention by the features of claim 1, a vane cell pump which is distinguished by low pulsation and favorable noise behavior being used as fixed displacement high pressure pump. A flow valve which can be acted on by an electromagnetic actuator is provided connected downstream of the vane cell pump, which flow valve, in conjunction with a pressure relief valve connected in between, easily makes it possible to adapt the delivery amount to the requirement needed for power assistance as a function of driving conditions and driving behavior, it being additionally possible, according to the invention, for the flow valve to be activated via the electromagnetic actuator during cold start operation in such a way that the pressure in the vane cell pump builds up as quickly as possible. The invention therefore advantageously combines the utilization of the flow valve to minimize the energy consumption and for improved cold start operation with a corresponding reduction in wear, this combination not requiring any structural outlay and substantially being based on the utilization of data recorded anyway for vehicle operation, with an appropriate software link.

In a particularly simple way, the flow valve in the solution according to the invention can be configured as a throttling valve, the closing direction of the throttling valve preferably corresponding to the direction of throughflow. Given appropriate spring loading in the closing direction it is accordingly possible to use weak springs, and the actuating forces which are to be produced electromagnetically are to be kept small irrespectively of this if the throttling valve is configured to be relieved of pressure in a further refinement according to the invention.

In a manner according to the invention, the electromagnetic actuator is activated under parameters defining cold start conditions in order to take cold start operation into consideration, said parameters usually being recorded during vehicle operation including internal combustion engine operation and it being possible to call them up both individually and linked to one another, it being possible to record as cold start parameters, in particular, the engine temperature, the outside temperature, the rotational speed of the engine, the standstill time of the engine and also further parameters, however.

Further details and features of the invention emerge from the claims and, furthermore, the invention will be explained in the following text using an exemplary embodiment:

FIG. 1 showing a diagrammatic representation of a hydraulic power steering system according to the invention, and

FIG. 2 showing a section through a flow valve configured as a throttling valve, as is to be used advantageously within the scope of the invention.

In the diagrammatic representation according to FIG. 1 of the hydraulic power steering system 1 according to the invention, 2 denotes the vane cell pump which is configured as a fixed displacement pump, is fed from a storage vessel 3 and delivers to a steering valve 4, downstream of which there is arranged an actuating device for the steering mechanism 5 unless it is integrated into the steering valve 4. The connections which lead from the steering mechanism 5 to the wheels are indicated at 6. Furthermore, the connection of the steering valve 4 to the steering wheel (not shown) is indicated by a steering shaft 7 (shown diagrammatically).

An expanding tube 8 lies in the connection of the vane cell pump 2 to the steering valve 4 as a pressure store and/or an element which damps pulses, and, furthermore, a flow valve 9 is arranged adjacent to the vane cell pump 2, said flow valve 9 being assigned an electromagnetic actuator 10, as shown in greater detail in FIG. 2.

In the junction between the vane cell pump 2 and flow valve 9, there is a branch 11 to a pressure relief valve 12 which is connected to the suction line 13 of the pump 2 via a line section 14. The pressure relief valve 12 is pressure controlled, to be precise as a function of the pressure prevailing in the line connection 15 between the flow valve 9 and steering valve 4, as indicated by the dash-dotted line 16. The return from the steering valve 4 to the storage vessel 3 takes place via a line 17 in which a cooler 18 is arranged.

Furthermore, a steering speed sensor 38 is indicated diagrammatically, by which the rotary speed of the steering shaft 7 is tapped off, and a speed sensor 19 is indicated diagrammatically, by which the driving speed of the vehicle is recorded and which may be formed, for example, by the speedometer of the vehicle. The respectively recorded signals are processed in a control unit 20, the connection being produced via what is referred to as a CAN bus 21, as indicated diagrammatically. Further control parameters which identify cold start operation of the vane cell pump and which are usually recorded anyway in conjunction with other vehicle functions are preferably processed by the control unit 20.

The supply of current to the actuator 10 is controlled by the control unit 20, the battery or some other current connection in the vehicle being provided, for example, as current source 22 and the line connections 23 leading from the control unit 20 to the actuator.

In the representation according to FIG. 2, a valve housing 24 is provided in which the inlet connection on the pump side is denoted by 25 and the connection which leads to the steering valve 4 is denoted by 26.

In the junction region between these connections 25 and 26, there lies the opening cross section 28 which is controlled via the throttling member 27 of the flow valve 9 configured as a throttling valve and into which the widened throttling head 29 of the throttling member 27 engages, the guide rod 30 being attached to said throttling head 29. The guide rod 30 extends as far as the region of the winding 31 of the electromagnetic actuator 10 and has an armature 32 in this end region. The guide rod 30 is drilled so as to be hollow, in such a way that the result is a pressure chamber 33 lying opposite the throttling head 29, with the consequence of pressure equalization, so that the throttling member 27 can be displaced in the opening direction in the case of relatively small adjusting forces to be applied via the actuator 10, the throttling head 29 entering the opening cross section 28 which widens in the opening direction. 34 denotes the relatively weak spring which loads the throttle member 27 in the closing direction.

During operation of the power steering system 1, the opening cross section 28 is opened by appropriate current being supplied to the actuator 10 in a manner controlled by the control unit 20, to such an extent as is expedient with regard to the booster action desired for driving reasons and under comfort aspects and with regard to the volumetric flow required for this purpose.

With regard to the reduced delivery capacity of the vane cell pump 2 under cold start conditions, the flow valve 9 which is configured as a throttling valve is activated under cold start conditions in such a way that the opening cross section 28, as shown in the drawing, is at least substantially closed above the throttling head 29, with the result that, without any or any substantial volumetric flow to the consumer, the steering valve 4 in this case, a pressure is initially built up with respect to the vane cell pump 2, which pressure presses the vanes of the vane cell pump 2 into their radially outer position on the cam ring. As a result, short circuit connections between the individual working chambers of the vane cell pump are quickly interrupted.

A method of operation of this type which serves substantially solely to build up pressure is required only for a very short time and therefore does not impair the power steering system with regard to immediate activation when starting the vehicle, and is achieved without extra expenditure on the construction via appropriate activation of the control valve 9 which, given appropriate activation by the control unit 20, additionally ensures that the vane cell pump 2 which operates as a fixed displacement pump conveys only that proportion of the delivery volume to the steering valve 4 or possibly a booster actuating unit which is required for driving and/or comfort reasons. 

1. A hydraulic power steering system for motor vehicles having a steering mechanism, a steering valve and a fixed displacement high pressure pump which has a connection to the steering valve in which a flow valve is situated whose opening cross section can be adjusted via an electromagnetic actuator with parameters which influence the steering behavior being taken into consideration, the high pressure pump being configured as a vane cell pump followed on the delivery side by the flow valve with a pressure relief valve connected in between, and the opening cross section of the flow valve is at least substantially closed during cold start opening of the vane cell pump.
 2. The hydraulic power steering system as claimed in claim 1, wherein the flow valve is configured as a throttling valve.
 3. The hydraulic power steering system as claimed in claim 2, wherein the closing direction of the throttling valve corresponds to the direction of throughflow.
 4. The hydraulic power steering system as claimed in claim 2, wherein the throttling valve is spring-loaded in the closing direction.
 5. The hydraulic power steering system as claimed in claim 2, wherein the throttling valve is configured to be relieved of pressure.
 6. The hydraulic power steering system as claimed in claim 1, wherein it is possible to actuate the electromagnetic actuator with parameters which limit cold start conditions of the vane cell pump being taken into consideration.
 7. The hydraulic power steering system as claimed in claim 6, wherein the engine temperature of the vehicle engine, the outside temperature, the rotational speed of the vehicle engine, the standstill time of the vehicle engine starting are provided as cold start parameters. 